In an electric power tool, there is used a secondary battery in view of portability and usability. In recent years, a nickel-cadmium or nickel-hydride battery is being replaced with a lithium-ion battery of lighter weight and higher capacity. However, the lithium-ion battery has a drawback, that is, a low tolerance for overcharging or overdischarging when compared to the nickel-cadmium or nickel-hydride battery.
For that reason, for a battery pack including a battery assembly in which a plurality of lithium-ion batteries is serially connected and received within a housing, there has been employed various apparatuses for each lithium-ion battery not to be overcharged or overdischarged.
FIG. 6 illustrates a schematic circuit configuration of a conventional battery pack 1 and a charger 2 disclosed in Japanese Patent Application Publication No. 2007-143284. Referring FIG. 6, schematic description will be made on the conventional example below.
The battery pack 1 is detachably connected to the charger 2 and charged. Further, the battery pack 1 is detachably installed in an electric power tool and supplies electric power thereto. As shown in FIG. 6, the battery pack 1 includes a battery assembly 4 having a plurality of, e.g., four secondary batteries 4A, 4B, 4C and 4D serially connected to each other; a pair of power terminals 5a and 5b connected to a positive and a negative terminal of the battery assembly 4, respectively; a first control circuit 7 for individually detecting voltages V1 to V4 across the secondary batteries 4A to 4D and, when at least one of the detected voltages V1 to V4 exceeds a first predetermined value Vth1, e.g., 4.2 V, outputting a charge control signal to the charger 2; and a second control circuit 8 for outputting a discharge stop signal when at least one of the voltages V1 to V4 are less than a second predetermined value Vth2, e.g., 2.0 V, which is lower than the first predetermined value Vth1.
Further, the battery pack 1 includes signal terminals 13b, 13d for outputting the charge control signal and the discharge stop signal, respectively; a second power terminal 5c which is connected to a positive terminal of the battery assembly 4 and to which a charging current is supplied from the charger 2; a protection element 6 for opening/closing a flow path of the charging current supplied to the battery assembly 4 via the second power terminal 5c; and a third control circuit 9 for individually detecting voltages V1 to V4 across the secondary batteries 4A to 4D and, when at least one of the voltages V1 to V4 is greater than a third predetermined value Vth3, e.g., 4.5 V, which is higher than the first predetermined value Vth1, opening the flow path of the charging current by operating the protection element 6. Herein, lithium-ion batteries are used as the secondary batteries 4A to 4D.
The protection element 6 includes a so-called non-restorable fusing resistor in which an electric current flowing in a heater resistor 6a melts a fuse element 6b to open a flow path of the electric current. The signal terminals 13b, 13d are included in a plurality of, e.g., six in the drawing, signal terminals 13a to 13f provided in a signaling connector 13. Herein, the signal terminal 13a is connected to ground, the signal terminal 13c is connected to ground via a temperature measuring element (thermistor) Th for detecting a temperature of the battery assembly 4.
The signal terminal 13f is also connected to ground via an identifying resistor element Rx and the signal terminal 13e is connected to the second control circuit 8. The identifying resistor element Rx′ is an element that has resistance depending on information relating to the battery assembly 4, e.g., the number, configuration, voltages, capacities or the like of the secondary batteries. The charger 2 and the electric power tool 3 can obtain the information relating to the battery assembly 4 by reading the resistance.
The first control circuit 7 includes a first detector 7a comparing the first predetermined value Vth1 with voltages V1 to V4 across respective secondary batteries detected by measuring electric potentials of positive electrodes of the secondary batteries 4A to 4D, and, when at least one of the voltages V1 to V4 exceeds the first predetermined value Vth1, outputting a high (H) level signal (i.e., an active high signal of an open-collector type output). The first control circuit 7 further includes a signal conversion circuit 7b converting the H level signal outputted from the first detector 7a into a charge control signal.
While the battery pack 1 is neither installed in the charger 2 nor the electric power tool (hereinafter, this state is referred to as “idle state”), the first detector 7a is powered from the battery assembly 4 and remains in a standby mode. For that reason, the first detector 7a is constituted by an integrated circuit (IC) in which an electric current consumption is extremely small, e.g., about 1 μA in the standby mode.
The signal conversion circuit 7b is supplied with power supply voltage VDD to operate while the battery pack 1 is installed in the charger 2 or the electric power tool. The signal conversion circuit 7b includes a switch element Q6 turned on/off based on a signal outputted from the first detector 7a, and a zener diode ZD6 connected to the power supply voltage VDD in parallel with the switch element Q6. When the signal outputted from the first detector 7a is low (L) level, the switch element Q6 turns off and an H level charge control signal (power supply voltage VDD) is outputted to the signal terminal 13b. Further, when the signal outputted from the first detector 7a is H level, the switch element Q6 turns on and an L level charge control signal is outputted to the signal terminal 13b. 
In addition, the zener diode ZD6 is provided to protect the switch element Q6 from a noise and a reverse withstanding voltage.
The second control circuit 8 includes a second detector 8a, a signal conversion circuit 8b, a delay circuit 8c, a drive circuit 8d, and a power supply control circuit 8e. The second detector 8a detects voltages V1 to V4 across respective secondary batteries 4A to 4D by measuring electric potentials of the positive electrodes thereof and compares them with a second predetermined value Vth2, and, when at least one of the voltages V1 to V4 is less than the second predetermined value Vth2, outputs an L level signal (i.e., an active low signal of an open-collector type output).
The signal conversion circuit 8b converts the L level signal outputted from the second detector 8a into a discharge stop signal. The signal conversion circuit 8b includes a switch element Q5 turned on/off based on a signal outputted from the second detector 8a and a zener diode ZD5 connected to the power supply voltage VDD in parallel with the switch element Q5. Further, the signal conversion circuit 8b is powered from the power, supply voltage VDD while the battery pack 1 is installed in the charger 2 or the electric power tool 3. Accordingly, when the signal outputted from second detector 8a is H level, the switch element Q5 turns on and an L level discharging stop signal is outputted to the signal terminal 13d and, when the signal outputted from the second detector 8a is L level, the switch element Q5 turns off and an H level discharging stop signal (power supply voltage VDD) is outputted to the signal terminal 13d. 
Further, the zener diode ZD5 is provided to protect the switch element Q5 from a noise and reverse withstanding voltage. The delay circuit 8c operates as an integral circuit by including a resistor R18 and a condenser C9 and lengthens a rising time period in the signal outputted from the second detector 8a. The power supply control circuit 8e also controls electric power supply from the battery assembly 4 to the second detector 8a. Furthermore, the drive circuit 8d drives the power supply control circuit 8e in response to a control signal inputted from the charger 2 or the electric power tool while the battery pack 1 is installed in the charger 2 or the electric power tool 3.
The third control circuit 9 includes a third detector 9a outputting an H level signal (an active high signal of a CMOS output) when at least one of the voltages V1 to V4 exceeds a third predetermined value Vth3, and a protection element drive circuit 9b for melting and disconnecting a fuse element 6b by making an electric current flown through a heater resistor 6a of the protection element 6 when the H level signal is outputted from the third detector 9a. 
More specifically, the third detector 9a detects voltages V1 to V4 across, respective secondary batteries 4A to 4D by measuring electric potentials at positive terminal of the secondary batteries 4A to 4D and compares the detected voltages V1 to V4 with the third predetermined value Vth3. Similar to the first detector 7a, the third detector 9a is also powered from the battery assembly 4 and remains in the standby mode during the idle state. Hence, the third detector 9a is constituted by an IC consuming an extremely small electric current, e.g., about 1 μA in the standby mode.
The protection element drive circuit 9b includes a resistor R26 and a switch element Q7. When the H level signal is outputted from the third detector 9a, the switch element Q7 turns on and an electric current flows in the heater resistor 6a of the protection element 6.
In the battery pack 1, ground of the signal terminal 13a is separated from ground of the power terminal 5b connected to a negative terminal of the battery assembly 4. Accordingly, it is possible to prevent an excessive discharging or charging current from flowing even if there is a connection error between power terminals 17a, 17b of the charger 2 or the electric power tool and the power terminals 5a, 5b and the second power terminal 5c. 
Meanwhile, the charger 2 includes a signaling connector 14 detachably connected to the signaling connector of the battery pack 1, and power terminals 7b and 7a detachably connected to the power terminal 5b and the second power terminal 5c of the battery pack 1, respectively. The charger 2 further includes a power supply circuit 21 for converting an alternative current (AC) electric power into a direct current (DC) electric power and outputting it to the power terminals 17a and 17b, and a charge control circuit 19 controlling a charge by adjusting an output of the power supply circuit 21.
The signaling connector 14 has signal terminals 14a to 14f connected to the signal terminals 13a to 13f of the signaling connector 13 of the battery pack 1, respectively. Further, the signal terminal 14a is connected to ground and the signal terminals 14d and 14f are pulled up via resistors 28 and 29 to power supply voltage VDD of the charge control circuit 19.
Next, a charging operation will be explained when the battery pack 1 is installed in the charger 2. When the battery pack 1 is installed in the charger 2, the second power terminal 5c, the power terminal 5b, and the signaling connector 13 are connected to the power terminals 17a and 17b, and the signaling connector 14, respectively. Accordingly, voltages of the signal terminals 14c and 14f change from the power supply voltage VDD into a voltage voltage-dividing the power supply voltage VDD by the pull-up resistor 28 and 29, the temperature measurement element (thermistor) Th, and the identifying, resistor element Rx. The charge control circuit 19 automatically detects installation of the battery pack 1 by detecting changes in the voltages of the signal terminals 14c, 14f and starts to charge.
Upon starting to charge, the charge control circuit 19 applies a control signal VD to the signal terminal 14e to operate the second control circuit 8. At the same time, the charge control circuit 19 reads information relating to the battery pack 1 from the voltage of the signal terminal 14f and also reads from the signal terminal 14c temperature of the battery assembly 4 detected by using the temperature measurement element Th. When the temperature of the battery assembly 4 is within a certain range and a charge control signal inputted to the signal terminal 14b is H level (voltages across every secondary batteries 4A to 4D is below the first predetermined value Vth1), the charge control circuit 19 operates the power supply circuit 21 to supply an electric current in a charging current path of the battery pack 1 (i.e., second power terminal 5c→protection element 6→battery assembly 4→power terminal 5b), thereby charging the battery assembly 4.
When one of the voltages across the secondary batteries 4A to 4D exceeds the first predetermined value Vth1 and a charge control signal inputted to the signal terminal 14b becomes L level, the charge control circuit 19 controls the power supply circuit 21 to reduce the charging current and makes transition to constant-voltage charge. The charge control circuit 19 reduces the charging current step by step whenever the charge control signal changes the H level to the L level. When the charging current goes below a threshold value, the charge control circuit 19 completes charging by stopping the power supply circuit 21 and stops application of the control signal VD to the signal terminal 14e. Accordingly; the second control circuit 8 stops, thereby suppressing an electric current consumption in the battery pack 1.
When the battery pack 1 is removed from the charger 2, the voltages of the signal terminals 14c, 14f increase to the power supply voltage VDD, which is detected by the charge control circuit 19. Since the signal terminals 14c, 14f of the signaling connector 14 are pulled up via the resistors R28, R29 to the power supply voltage VDD in the charger 2, removing the battery pack 1 can be detected only by stopping application of the control signal VD to the signal terminal 14e. Further, if a pull-up resistor is provided in the battery pack 1, the control signal VD needs to be applied as long as a voltage is not applied to a signal terminal which is additionally provided.
In the above conventional example, four kinds of safety functions operate upon charging. Firstly, when at least one of the voltages across the secondary batteries 4A to 4D exceeds the first predetermined value Vth1 in the first control circuit 7, as described above, the first control circuit 7 informs the charge control circuit 19 of the charger 2 of the voltage excess by changing the charge control signal from H level to L level. Accordingly, the charge control circuit 19 controls the power supply circuit 21 to reduce the charging current.
Secondly, temperatures of the secondary batteries 4A to 4D are detected by using the temperature measurement element Th and, when the detected temperatures exceed a certain value (70° C.), the charge control circuit 19 stops charging. Thirdly, the charge control circuit 19 monitors voltage of the second power terminal 5c and, when the voltage exceeds a certain value (17.5 V), stops charging. Fourthly, when at least one of the voltages across the secondary batteries 4A to 4D exceeds the third predetermined value (4.5 V), the third control circuit 9 operates the protection element 6 to disconnect (melt) a charging current path.
As another conventional example, there is disclosed a battery pack in Japanese Patent Application Publication H10-12283. The battery pack in accordance with the above another conventional example detects temperatures of a group of batteries (battery assembly) by using temperature sensors. Such a battery pack determines an internal voltage of each battery based on each battery voltage measured by a voltage measurement circuit, a charging/discharging current measured by a current measurement circuit, and the temperatures detected by the temperature sensors. The battery pack further detects a residual capacity of the group of batteries based on each internal voltage.
If a lithium-ion battery is used as a secondary battery of the battery pack, the higher a charging voltage is, the larger a charging capacity but the worse a life span or safety of the battery is. Reversely, if the charging voltage is lowered, the capacity becomes smaller but the life span or safety is better. Further, the lithium-ion battery has an upper limit in the charging voltage depending on the temperature (battery temperature). If temperature of the battery is not within a proper range, it adversely influences on the life span thereof. Particularly, when the battery temperature exceeds a proper range, there occurs a problem that the life span of the battery becomes drastically shorter unless the upper limit of the charging voltage is lowered.
Therefore, in order to prevent the life span of the battery from reducing while the capacity remains as much as possible, it is, needed to adjust the charging voltage to a proper level depending on the battery temperature. In this regard, the charger 2 stops charging when the temperatures of the secondary batteries detected by the temperature measurement element exceed a threshold value in the former conventional example. In the latter, the internal voltages of the batteries are corrected based on the temperatures of the batteries. Not in both cases, it was to adjust the charging voltage based on the temperature of the battery.